Putative genetic differences have been found in 5-HTP and dopamine decarboxylase by physicochemical techniques. The enzymes in liver and kidney have been found to differ, sometimes in the same individual. In extensive tests, no evidence that 5-HTP and DOPA decarboxylase activity is due to different enzyme molecules has been found. No electrophoretic differences were found for tyrosine hydroxylase between strains of mice tested.